Note: Descriptions are shown in the official language in which they were submitted.
DEVICE AND METHOD FOR DECREASING RADIATIVE HEAT FLUX OF INFRARED ENERGY
Applicant claims the benefit of U.S. Provisional Application Serial No.
62/423,520
filed November 17, 2016.
BACKGROUND
[001] It is widely accepted that infrared energy is superior to other forms of
heat energy
for certain industrial curing and drying processes. In the past 10-15 years,
infrared energy
generated from fuels such as butane, propane and natural gas has also become
popular for use
in outdoor grills and for indoor grills used in restaurants. All of these gas
fired grills depend upon
the combustion of a gaseous fuel for the generations of infrared energy. It is
quite simple to
achieve radiative heat flux levels high enough to sear meat and to cook it
quickly. Such meats
include steaks, chops, hamburgers, ribs and small roasts. A hamburger with a
diameter of about
inches (12.7 cm) and 1/2 inch (1.3 cm) thickness weighing about .40 pounds
(0.18 kg) can be
broiled to an internal temperature of 160 F (71 C) in less than 10 minutes.
[002] All gas burners that depend on a venturi or an air injector tube to
introduce
primary air for combustion have a minimum fuel input for low fire. This
restriction limits most
infrared energy types of grills for use in slow cooking over an extended
period of time because
the limitation of the turn down ratio does not allow the infrared energy to be
reduced to a level
required ¨ less than a total emissive power of about 1000 BTU/HRFT2---for
traditional slow
cooking, barbecuing, and smoking that can take up to 12-14 hours or more.
[003] A growing interest in slow cooking and smoking in recent years has
spawned a
rapidly growing sector of the outdoor cooking equipment industry, a sector
which includes
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traditional smokers as well as kamado-style ceramic cookers and pellet grills.
However, these
types of cookers are not capable of reaching the high searing temperatures of
infrared grills. An
apparatus that can reduce the total emissive power of infrared grills and can
be installed on and
removed from a grill easily would make infrared grills far more versatile by
enabling consumers
to slow cook and smoke as well as sear on the same piece of cooking equipment.
SUMMARY OF THE INVENTION
[004] The present invention is a method and device utilizing infrared energy
for heating
objects, while providing energy control and enabling a decrease radiative heat
flux (or intensity)
of the infrared energy. An infrared emission device providing reduction of
radiative heat flux or
intensity from a primary emitter according to the invention may comprise a
heat source, a
primary emitter that emits infrared radiation of a first wavelength, and a
secondary emitter that
is spaced apart from the primary emitter. The secondary emitter receives
infrared radiation
emitted from the primary emitter and emits infrared radiation. The secondary
emitter is
constructed and arranged to emit infrared radiation having a wavelength that
is longer than the
infrared radiation of the first wavelength.
DESCRIPTION OF THE DRAWINGS
[005] Figure 1 is a sectioned side elevation of a device according to an
embodiment of
the invention.
[006] Figure 2 is an exploded view showing elements of a device according to
an
embodiment of the invention.
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[007] Figure 3A is a perspective, but exploded, view showing elements of a
device
according to an embodiment of the invention.
[008] Figure 3B is a perspective view showing the elements of a device
according to the
embodiment of the invention of Figure 3A in relationship for use.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[009] This invention includes a method and device for diminishing radiative
heat flux (or
intensity) of infrared energy. Devices for accomplishing the diminished
radiative heat flux (or
intensity) of infrared energy according to the invention preferably reduce the
infrared energy
emitted from a first or primary emitter 2 to below a total emissive power of
936 BTU/HR FT2 for
all wavelengths, wherein more than 50% of the wavelengths are in excess of 8
microns. The
infrared radiative heat flux limiter is referred to herein as a secondary
emitter 4.
[010] A preferred embodiment of the invention comprises a plate (secondary
emitter 4)
that is interposed between a primary emitter 2 of infrared energy and the
energy absorbing
object(s) 10. Fig. 1. By way of example, the energy absorbing objects may be
food that is
supported on a support member 12. The invention decreases the radiative heat
flux (or intensity)
from the primary emitting source by absorbing infrared energy emitted by the
primary emitter
and reradiating the infrared energy at longer wavelengths, based on the
secondary emitter's
radiant properties, thereby decreasing the temperature and/or decreasing
emissivity of the
secondary emitter from that of the primary emitter. The intensity of the
energy transmitted to
the object, such as food, is decreased. In some applications, both temperature
decrease and
decreased emissivity are employed.
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[011] The materials from which the primary emitter 2 and the secondary emitter
4 may
be constructed included metal, glass, ceramic glass, ceramic and other
material that has the
ability to operate at temperatures up to approximately 500 F (260 C). The
form of the plate
may be flat or have a small curvature. The plate may be fabricated with side
walls in a pan-like
structure to add rigidity. Support ridges may be pressed or otherwise formed
in the plate to
increase rigidity and diminish warping resulting from expansion during
heating. The plate may
have a plurality of apertures formed in a surface of the plate that allow
passage of some infrared
radiation from the primary emitter(s) but block other infrared radiation.
[012] This invention is believed to be of particular benefit when the fuel
provided for
combustion and heat generation is a combustible gas, such as propane, butane
or natural gas.
Gas burners inherently have a limit with regard to reducing heat output. That
is, such burners
have a turn down limitation that is associated with combustibility of the gas-
air mixture. When
this limitation is exceeded, the burner's flame is extinguished, and
combustion and energy
generation is terminated. The invention diminishes the intensity, or radiative
heat flux, of
infrared energy when low levels of such energy are desirable in an application
but cannot be
attained by adjustment of the fuel input to the burner.
[013] As shown in the embodiment of Fig. 1 and Fig. 2, a housing 14 has a
burner 16 and
a combustion plenum 18. A gas inlet 20 and regulator 22 are provided.
Combustible gas is mixed
with air and ignited at sufficient temperature. Air may be provided through
orifice 24. The
burner emits flame and products of combustion into the combustion plenum. An
exhaust port
26 may be provided. Also emitted is infrared energy. The primary emitter 2
receives the infrared
energy at a first, or lower, surface 8 of the primary emitter. The primary
emitter emits infrared
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energy of a first wavelength 30 from a top, or upper, surface 6 of the primary
emitter. The
secondary emitter 4 receives the infrared energy from the primary emitter on a
lower surface of
the secondary emitter 33. The secondary emitter is constructed and arranged to
emit at least
50% of its infrared energy from the top surface 28 at a wavelength 32 that is
longer than the first
wavelength emitted by the primary emitter.
[014] In a preferred embodiment, the device according to the invention is a
grill. A grill
may be built generally in accordance with the grill shown in Best, U.S. Patent
No. 6,114,666, and
modified with a secondary emitter according to the present invention. An
example of the effect
of the invention on a grill so constructed is as follows: A test indicates
that the temperature of
the primary emitter on low fire setting was 540 F (271 C), while the
temperature of the
secondary emitter was 320 F (160 C). Output radiation flux density based on
the Stefan-
Boltzman equation is stated as follows for this application.
Q = .173 x 10-8 xexA (Ti4 ¨ T24)
Q = BTU\HR
. 173 x 10-8= Stefan-Boltzman Constant
e=emissivity
A= Area\FT2
T14 = R4 (emitting surface temperature)
T24 = R4 (absorbing surface temperature)
Note: When computing radiative heat flux use only T14.
[015] Solving the above equation for the primary emitter with a temperature of
520 F
(271 C) and an emissivity of .92 indicates that the total emissive power of
the primary emitter is
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1575 BTU/HR FT2 and for the secondary emitter with a temperature of 320 F (160
C) and
emissivity of .92 indicates the total emissive power of the secondary emitter
is 582 BTU/HRFT2.
[016] The present invention according to a preferred embodiment comprises a
secondary emitter 4. The secondary emitter may be a metal plate in one
embodiment. The metal
plate may have side walls (pan-like) for support, with walls about the entire
perimeter of the
secondary emitter. A first side (lower surface) of the secondary emitter 33
that faces the primary
emitter is an absorbing side that absorbs infrared energy from a primary
emitter. The obverse
side 28 of the secondary emitter, which may be a metal plate, emits infrared
energy 32 absorbed
by the first side of the metal plate 33. Fig. 2. Heat tolerant or heat
resistant coatings of different
types may be applied on one or both sides to vary the emissivity to achieve
the desired result of
reducing the emissive power of the primary emitter. For example, the coatings
may be ceramic,
porcelain or high temperature paint that will withstand the operating
temperatures.
[017] The secondary emitter 4 is preferred to be spaced apart from the primary
emitter
2. For example, the secondary emitter may be spaced 1/2 inch (1.3 cm) to 5
inches (12.7 cm) from
the primary emitter. In one embodiment, the secondary emitter is supported by
legs 34 having
a selected length that maintain the spaced apart relationship between the
primary and secondary
emitter. Fig. 3. In another embodiment, the secondary emitter is placed
directly on a cooking
grate of a grill. In that construct, the grates maintain a spaced apart
relationship between the
primary emitter and the secondary emitter. Other types of mechanical frames of
various
construct may be used. The secondary emitter may have perforations or other
apertures formed
therein through which a portion of the infrared energy emitted from the
primary emitter passes
directly to the absorbing object(s). The apertures may be constructed to be
closed or partially
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closed. In one embodiment, the secondary emitter is formed with two plates,
each having
apertures. The apertures may be aligned to show that the apertures are open,
permitting the
passage of energy through them. One plate may be moved relative to the second
plate to close,
or partially close the apertures.
[018] A material used for most of the experimental secondary emitter plates is
metal,
which may be stainless steel. In other related experiments, various coatings
have been applied
to metal other than stainless steel. Other substrates, both coated and
uncoated, that exhibit the
required emissivity properties have been employed with equal success at
required operating
temperatures.
[019] When the invention is used as preferred to lower radiative heat flux
from the
primary emitter 2 in a cooking or broiling application, means is provided to
support the food 10
above the secondary emitter 4. Fig. 1; Fig. 2. The support member 12 is
positioned above and is
spaced apart from the secondary emitter. The construct of the support member
(i.e. rack or
grate) may vary according to the application, such as a particular grill
construct. Fig. 3. The
support member is characterized by the grate having openings that allow
infrared energy to pass
through the openings. More than 30% of the surface area of the support member
is open, and
more preferably, at least 50% of the surface area of the support member is
open. This is
contrasted with the secondary emitter, wherein the infrared energy is emitted
from the plane of
the secondary emitter and openings allow infrared energy from the primary
emitter to pass
through. Therefore, openings in the secondary emitter are less than 30% of the
surface area in
most applications, and the secondary emitter may not have any openings in the
surface.
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[020] In some applications, the support member for food may be attached to the
grill
body, or the food support may be attached to the frame of the secondary
emitter, with the plane
of the support member generally parallel to the plane of the primary emitter
and the secondary
emitter.
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